It is known that present stored-program control systems present memories organized in a hierachic structure, providing fast-access memories for on-line programs and data (main memories), followed by other memories, generally with slower access, for programs and data for less immediate and frequent use (mass memories). These memories often perform also as auxiliary memories for the main memories, that is they contain also semi-permanent data and on-line programs necessary to allow the control system to be put again into the usual service when a failure occurs in the main memories.
Till now mass memories have been usually realized by disk units, magnetic tapes or drums, because, owing to the state of the art, these solutions alone could ensure large storage capacity at reduced cost.
However, magnetic memories present a number of inconveniences:
they can not attain sufficiently high operating speed, chiefly fast access time;
they can not ensure a sufficiently high "system availability" (intended as probability of finding the system operating at any moment), owing to the frequent interventions necessary to maintain the efficiency of the units; this feature is emphasized by the fact that the magnetic units have moving mechanical parts that require an initial running-in and present wear phenomena that can also require preventive maintenance.
For these reasons the studies meant to realize memories of different types mainly for small and middle capacity (for instance up to 10 million words) have proved to be of main importance; owing to the development in the techniques able to realize solid-state components, the studies have been directed towards components of very high-scale integration, and more particularly towards chargecoupled devices.
A memory of this type with operating characteristics very similar to those of a disk unit is already commercially available.
A solid-state memory like this one intrinsically presents high operating speed as well as good reliability and easy-maintenance characteristics; moreover it presents good modularity which is why, to begin with, rather small units may be used that can be increased according to the requirements.
However, this known memory presents a number of inconveniences that make it not so apt to be used in telescommunications systems control: more particularly it provides no error self-correcting possibility and is organized by "btyes", that is by 8-bit words.
But in telecommunications applications the control system must be in service continuously: then it is important for the mass memory to be provided with self-correcting means preventing the system from being put out of service during the time necessary to detect the cause of the error and to remedy same; self-correction provides an efficient protection of the stored data, so that said data do not get lost and can be used by a possible auxiliary unit put into service by a reconfiguration system.
Moreover, for velocity purposes, the system must operate on words of 16 bits at least.